#include<iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <stack>
using namespace std;

struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(NULL) {}
};
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if (l1 == NULL && l2 == NULL)return NULL;
        if (l2 == NULL)return l1;
        if (l1 == NULL)return l2;

        ListNode* head,*p;
        if (l1->val < l2->val)
        {
            head = l1;
            l1 = l1 -> next;
            p = head;
        }
        else 
        {
            head = l2;
            l2 = l2->next;
            p = head;
        }
        while (l1 != NULL && l2 != NULL)
        {
            if (l1->val < l2->val)
            {
                p->next = l1;
                l1 = l1->next;
                p = p->next;
            }
            else
            {
                p->next = l2;
                l2 = l2->next;
                p = p->next;
            }
        }
        if (l1 != NULL)p->next = l1;
        else p->next = l2;
        return head;
    }
};
int main()
{
    Solution sol;
}